1. Field of the Invention
The present invention relates to an optical disc apparatus for optically processing information over an information storage medium such as an optical disc.
2. Description of the Related Art
In recent years, as optical disc apparatuses which process information over an information storage medium, optical disc apparatuses have been actively used. On an optical disc as an information storage medium, spiral-shaped or concentric-shaped information tracks named grooves are formed. In the optical disc apparatus, by forming physical holes, changing magnetic characteristics, or changing metal state at an information track or between information tracks, information is written (recorded) or read (reproduced).
Generally, in the optical disc apparatus, as an optical disc is rotated, a signal surface on which an information signal is recorded vertically moves due to machining accuracy, rotation accuracy, and so forth. To precisely read the information signal, the optical disc apparatus is generally provided with a focusing function for keeping the distance between an objective lens (which focuses a light beam of a light source on a target) and an information storage medium constant. In other words, a focus servo system is used which drives the objective lens corresponding to the vertical movement of the signal surface so that the objective lens focuses a laser beam irradiated by an optical head on the signal surface of the information storage medium.
As described in the Japanese Patent Application Laid-open No. HEI4(1992)-49530, a light beam reflected from the information storage medium is detected by an optical head. The optical head outputs a detection signal. The detection signal is converted into a digital signal by an A/D converter. Corresponding to the resultant signal, the objective lens is driven by a digital servo system.
The output range of the detection signal is limited. Thus, when the optical disc apparatus is operated, the objective lens should be aligned in such a range. To do that, the optical disc apparatus is provided with a circuit which reciprocally moves the objective lens over the optical disc using triangular pulses or the like so as to seek a focus position.
In addition, the optical disc apparatus is provided with a function for causing the focused light spot to target track on the information storage medium. This function is referred to as the tracking function. The tracking function is accomplished in the following manner. A detection signal (representing the distance between a target track on the information storage medium and the light spot) is generated corresponding to the light beam reflected from the information storage medium. The detection signal is converted into a digital signal by an A/D converter. Corresponding to the digital signal, the objective lens is driven by a digital servo system.
As described above, in the conventional optical disc apparatus, both the detection signal for use in the focus servo system and the detection signal for use in the tracking servo system are converted into digital signals by the A/D converters.
However, in the optical disc apparatus, an optical disc can be replaced with another one. Thus, the level of signal varies corresponding to the reflection ratio, the shape of guide grooves, and emboss data (sector mark and so forth). In addition, when the focus servo operation or tracking servo operation is performed, a small variation of signal should be read in the vicinity of a servo point. Thus, the A/D converters should have wide dynamic range and high accuracy. Moreover, when one digit deviates in the A/D converters, the focus servo system is adversely affected. As a result, the servo function may not correctly work.